Maraging steel alloy and method of manufacturing same

Abstract

To provide maraging steel alloys having improved microstructures.SOLUTION: Provided are maraging steel alloys having improved microstructures. Some variations provide maraging steel alloys including a base maraging steel alloy, a grain refiner, and optionally a strengthening element. The base maraging steel alloy is surface-functionalized with the grain refiner. Other variations provide a method of manufacturing maraging steel including the steps of: mixing a base maraging steel alloy with a grain refiner to obtain a maraging steel mixture; melting the maraging steel mixture; and solidifying the maraging steel mixture to form an equiaxed microstructure.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] Exemplary embodiments of the present disclosure relate generally to maraging steel alloys, and more particularly to methods of forming maraging steel alloys. [Background technology]

[0002] Maraging steel is a low-carbon, ultra-high-strength steel that possesses both high tensile strength and high fracture toughness. Previous maraging steel alloys have been difficult to process using additive manufacturing. Currently, only one high-strength steel alloy is available for additive manufacturing, but this alloy suffers from anisotropic material properties and low fracture toughness due to a highly columnar grain structure. Both the anisotropic material properties and low fracture toughness are typical of many additive steel alloys that tend to solidify in a columnar structure. Iron's crystalline structure generally has highly anisotropic elastic constants, which lead to poor overall material properties, including fatigue and fracture toughness. The highly anisotropic elastic constants, resulting from large differences in Schmid factors between differently oriented grains, lead to large local stresses and strains at grain boundaries under typical, modest loads, causing premature fracture and limiting the achievable strength and toughness. Summary of the Invention [Problem to be solved by the invention]

[0003] Provided herein are maraging steel alloys and methods for their manufacture. The maraging steel alloys contain grain growth inhibitors that produce unique microstructures during additive manufacturing, enabling the production of alloys that were previously difficult to process. These grain growth inhibitors can be targeted to specific alloy compositions and can be incorporated at higher concentrations by incorporating the grain growth inhibitors during the additive manufacturing process. Additive manufacturing has previously been limited to weldable or castable alloys. The present disclosure removes this limitation and can result in a wrought-like microstructure rather than the typical columnar structure produced by additive manufacturing. The present disclosure can enable the additive manufacturing of a variety of high-strength, difficult-to-process maraging steel alloys by utilizing grain growth inhibition to produce an equiaxed microstructure that can reduce cracking propensity and produce more isotropic material properties. [Means for solving the problem]

[0004] In some embodiments, a maraging steel alloy may be provided, the maraging steel alloy including a base maraging steel alloy, a grain growth inhibitor dispersed throughout the base maraging steel alloy, and optional strengthening elements. The base maraging steel alloy may be surface functionalized with the grain growth inhibitor.

[0005] In some embodiments, the base maraging steel alloy can include aluminum, cobalt, molybdenum, nickel, titanium, or a combination thereof, and has a tensile strength greater than 1300 MPa. In some embodiments, the grain growth inhibitor can include titanium, zirconium, boron, aluminum, tantalum, tungsten, carbon, niobium, cerium, or a combination thereof, as a pure metal, oxide, hydride, carbide, nitride, intermetallic compound, boride, or a combination thereof. In some embodiments, the grain growth inhibitor can include TiB2, CeO2, TiN, NbC, or a combination thereof. In some embodiments, the strengthening element can include nickel, aluminum, cobalt, chromium, molybdenum, carbon, manganese, niobium, zirconium, titanium, or a combination thereof.

[0006] In some embodiments, the grain growth inhibitor may comprise about 0.01% to about 10% by volume of the maraging steel alloy. In some embodiments, the maraging steel alloy may comprise an equiaxed microstructure. In some embodiments, the equiaxed microstructure may comprise a plurality of grains less than 1 mm in diameter, and in some embodiments, the equiaxed microstructure may comprise a uniform grain pattern along the x and y directions. In some embodiments, the equiaxed microstructure may form a scalloped pattern.

[0007] Also provided herein is a method of producing a maraging steel, in some embodiments, the method includes mixing a base maraging steel alloy with a grain growth inhibitor to provide a maraging steel mixture, melting the maraging steel mixture, and solidifying the maraging steel mixture to form an equiaxed microstructure.

[0008] In some embodiments, solidifying the maraging steel mixture may include solidifying a first layer of the maraging steel mixture along a single axis and then solidifying an adjacent layer of the maraging steel mixture along the same axis.

[0009] In some embodiments, the base maraging steel alloy can exist as a powder when mixed with the grain growth inhibitor. In some embodiments, the base maraging steel alloy and the grain growth inhibitor can have a lattice strain of less than 5%. In some embodiments, the base maraging steel alloy and the grain growth inhibitor can have an atomic density difference of less than 25%.

[0010] In some embodiments, the base maraging steel alloy can include aluminum, cobalt, molybdenum, nickel, titanium, or a combination thereof, and the base maraging steel alloy has a tensile strength greater than 1300 MPa. In some embodiments, the grain growth inhibitor can include titanium, zirconium, boron, aluminum, tantalum, tungsten, carbon, niobium, cerium, or a combination thereof, as a pure metal, oxide, hydride, carbide, nitride, intermetallic compound, boride, or a combination thereof. In some embodiments, the base maraging steel alloy can further include strengthening elements including nickel, aluminum, cobalt, chromium, molybdenum, carbon, manganese, niobium, zirconium, titanium, or a combination thereof. In some embodiments, the grain growth inhibitor can include TiB2, CeO2, TiN, NbC, or a combination thereof. In some embodiments, the grain growth inhibitor can comprise about 0.01% to about 10% by volume of the maraging steel alloy.

[0011] The foregoing summary has been presented merely for the purpose of summarizing some exemplary embodiments and providing a basic understanding of some aspects of the present disclosure. Accordingly, it will be understood that the above-described embodiments are merely examples and should not be construed to narrow the scope or spirit of the present disclosure in any way. It will be understood that the scope of the present disclosure encompasses numerous potential embodiments, some of which are further described below, in addition to the embodiments summarized herein.

[0012] Having generally described certain exemplary embodiments of the present disclosure, reference is now made to the accompanying drawings, which are not necessarily drawn to scale. [Brief explanation of the drawings]

[0013] [Figure 1] 1 illustrates an exemplary grain growth inhibitor disposed along a base maraging steel alloy according to certain exemplary embodiments described herein. [Figure 2A] 1 illustrates exemplary semi-passive solidification control including nucleation control according to some exemplary embodiments described herein, in which a crystal growth inhibitor acts as a nucleation site resulting in equiaxed grains in the final solid material. [Figure 2B] 1 illustrates exemplary semi-passive solidification control including nucleation control according to some exemplary embodiments described herein, in which a crystal growth inhibitor prevents the unstoppable growth of individual dendrites, resulting in equiaxed grains in the final solid material. [Figure 2C] 1 illustrates an exemplary additive manufacturing of a functionalized maraging steel alloy according to certain exemplary embodiments described herein. [Figure 3] 1 illustrates an exemplary semi-passive solidification control involving a peritectic reaction of a dissolved crystal growth inhibitor upon cooling, resulting in the crystal growth inhibitor-induced formation of dispersoids, according to certain exemplary embodiments described herein. [Figure 4]The solidification of the melt without significant migration of the incorporated crystal growth inhibitors demonstrates exemplary semi-passive solidification control according to some exemplary embodiments described herein, which can orient the crystal growth inhibitors into a three-dimensional structure that is repeated throughout the final solid material. [Figure 5A] 1 illustrates exemplary semi-passive solidification control including thermodynamic control according to some exemplary embodiments described herein, in which a crystal growth inhibitor reacts within the melt and the reaction enthalpy is utilized to control heat flow during solidification. [Figure 5B] 1 illustrates exemplary semi-passive solidification control involving thermodynamic control in accordance with some exemplary embodiments described herein, in which a crystal growth inhibitor or its reaction product is driven to the surface where it vaporizes, removing heat from the system. [Figure 6A] 1 illustrates exemplary semi-passive solidification control including conductivity or emissivity control according to some exemplary embodiments described herein, where a crystal growth inhibitor driven to the surface forms a layer with a different conductivity or emissivity than the underlying material. [Figure 6B] 1 illustrates exemplary semi-passive solidification control, including conductivity or emissivity control, according to some exemplary embodiments described herein, in which the grain growth inhibitor remains distributed and alters the conductivity of the melt and the final solid material. [Figure 7A] 1 illustrates exemplary semi-passive coagulation control including contaminant removal and expulsion to a surface according to certain exemplary embodiments described herein. [Figure 7B] 1 illustrates an exemplary semi-passive coagulation control involving reaction with contaminants, where the reacted contaminants remain within the solid, according to certain exemplary embodiments described herein. [Figure 8] 1 illustrates exemplary surface melting of a functionalized maraging steel alloy particle according to some exemplary embodiments described herein, where heat is applied and the grain growth inhibitor reacts with the surface to form a melt in less than 100% of the particle volume. [Figure 9]FIG. 1 illustrates an exemplary formation of a layered composite structure according to some exemplary embodiments described herein, where a functionalized maraging steel alloy with two different types of grain growth inhibitors results in the separation of different grains resulting in a layered structure. [Figure 10A] 1 illustrates various microstructures, including an equiaxed microstructure, according to certain exemplary embodiments disclosed herein. [Figure 10B] 1 illustrates various microstructures, including an equiaxed microstructure, according to certain exemplary embodiments disclosed herein. [Figure 10C] 1 illustrates various microstructures, including an equiaxed microstructure, according to certain exemplary embodiments disclosed herein. [Figure 11-1] 11A, 11B, and 11C illustrate various microstructures, including an equiaxed microstructure, according to certain exemplary embodiments disclosed herein. [Figure 11-2] 11D, 11E, and 11F illustrate various microstructures, including an equiaxed microstructure, according to certain exemplary embodiments disclosed herein. [Figure 12] 1 is a graph of hardness versus aging temperature showing typical data of heat treatment response obtained for functionalized maraging steel alloys according to certain exemplary embodiments disclosed herein; [Figure 13] 1 is a flowchart of an exemplary method of manufacturing a maraging steel alloy according to certain exemplary embodiments described herein. [Figure 14] 1 illustrates an exemplary application of a functionalized maraging steel alloy in an aircraft according to certain exemplary embodiments described herein. DETAILED DESCRIPTION OF THE INVENTION

[0014] Generally, embodiments of the present disclosure provided herein include maraging steel alloys and methods for making maraging steel alloys. More specifically, maraging steel alloys having improved microstructures and methods for making the same are provided. Certain embodiments of the present disclosure will now be described more fully below with reference to the accompanying drawings, in which some, but not all, embodiments of the present disclosure are shown. Indeed, these disclosures may be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will satisfy applicable legal requirements. Like numbers refer to like elements throughout.

[0015] "Anisotropic" refers to a component that has at least one chemical or physical property that is directionally dependent. An anisotropic component has some variation in the measurable property when measured along different axes. The property may be a physical (e.g., geometric) property, a chemical property, or both. A property that varies across multiple axes may simply be the existence of mass; for example, a perfect sphere is considered geometrically isotropic, while a cylinder is geometrically anisotropic. The amount of variation in a chemical or physical property may be about 5%, 10%, 20%, 30%, 40%, 50%, 75%, 100%, or more.

[0016] "Equiaxed grains" or "equiaxed microstructure" refers to grains or microstructures containing such grains whose dimensions are the same in different axes. "Grain boundaries" refer to points where the crystallographic orientation of the lattice changes. Grain dimensions can be measured from grain boundary to grain boundary.

[0017] A "scallop pattern" refers to at least a portion of a microstructure in which curved grain boundaries occur repeatedly along one axis and rows of such curved grain boundaries form along a second, perpendicular axis. A typical scallop pattern can be seen in Figure 10C of the present disclosure.

[0018] The term "by weight," unless otherwise indicated, refers to the weight percentage of a particular component in the total weight of a compound, composition, layer, or other applicable part. The term "by volume," unless otherwise indicated, refers to the volume percentage of a particular component in the total volume of a compound, composition, layer, or other applicable part. Weights and volumes are measured by known methods, such as by weighing the components or using methods such as ICP-OES or ICP-AES.

[0019] "Maraging steel alloy" refers to a class of low-carbon, ultra-high-strength steels. The primary alloying element is usually nickel, but maraging steel alloys can also contain cobalt, molybdenum, titanium, aluminum, and other trace elements. A typical maraging steel composition contains about 10 to about 25 wt.% nickel, about 0 to about 20 wt.% cobalt, about 0.1 to about 15 wt.% molybdenum, about 0.01 to about 10 wt.% titanium, and about 0.01 to about 10 wt.% aluminum, with the remainder being iron and, optionally, carbon. For example, a typical maraging steel composition contains about 15 to about 25 wt.% nickel, about 5 to about 15 wt.% cobalt, about 1 to about 10 wt.% molybdenum, about 0.1 to about 2 wt.% titanium, and about 0.01 to about 1 wt.% aluminum, with the remainder being iron and, optionally, carbon. For example, a typical maraging steel composition may include about 17 to about 19 wt. % nickel, about 7 to about 12.5 wt. % cobalt, about 3 to about 5.2 wt. % molybdenum, about 0.15 to about 1.6 wt. % titanium, and about 0.05 to about 0.25 wt. % aluminum, with the remainder being iron and, optionally, carbon. In some embodiments, manganese can be added to replace nickel or reduce the amount of nickel in the alloy. For example, the alloy may include about 9 to about 15 wt. % manganese. In some embodiments, the maraging steel alloy may be a cobalt-free maraging steel. For example, the maraging steel alloy may include about 18.9 wt. % nickel, 4.1 wt. % molybdenum, and 1.9 wt. % titanium, with the remainder being iron. Maraging steel alloys generally have high tensile strengths, for example, greater than about 1000 MPa, such as greater than about 1300 MPa, or even greater than 1600 MPa, although lower strengths are possible in alloys that are still considered maraging steel alloys. Tensile strength is measured using known methods, such as the ASTM E8 test method.

[0020] The term "base maraging steel alloy" refers to a maraging steel alloy that does not contain a grain growth inhibitor. The base maraging steel alloy may contain additional elements, such as strengthening elements, in addition to the typical maraging steel alloy composition described above, although such elements would typically be dissolved in the alloy in trace amounts. The base maraging steel alloy may then be functionalized with a grain growth inhibitor to form a functionalized maraging steel alloy having an equiaxed microstructure. The grain growth inhibitor may be the same element as that of the base maraging steel alloy, but is added during additive manufacturing to form the equiaxed microstructure. The composition of the base maraging steel alloy may be similar to the typical maraging steel composition described above, except for the additional elements that reduce the remaining iron and / or carbon.

[0021] "Grain growth inhibitor" refers to an additive that is incorporated into the base maraging steel alloy to form an equiaxed microstructure. The lattice matching of the grain growth inhibitor and the base maraging steel alloy can lower the critical amount of undercooling required for nucleation, resulting in equiaxed growth and reducing the columnar growth observed when the grain growth inhibitor is not incorporated into the base maraging steel alloy. The grain growth inhibitor may be nanoparticles, microparticles, or a combination thereof.

[0022] "Strengthening elements" refers to additional elements that can be added to a maraging steel alloy to form a base maraging steel alloy. These strengthening elements can be trace elements dissolved in the base maraging steel alloy.

[0023] "Powder" or "micropowder" refers to a state of fine, free particles. Powder materials are common feedstocks for powder metallurgy (or similar) processes, including, but not limited to, additive manufacturing, injection molding, and pressing and sintering applications. As intended herein, "powder material" refers to any powdered ceramic, metal, polymer, glass, or composite material, or combinations thereof. In some embodiments, the powder is a metal or metal-containing compound. The base maraging steel alloy may be provided as a powder prior to melting with one or more grain growth inhibitors. The powder size is typically between about 1 micron and about 1 mm, but in some cases may be as large as about 1 cm.

[0024] Powdered materials may be in any form in which individual particles can be reasonably distinguished from the whole. Powdered materials are not always observed as free powders, but may exist as pastes, suspensions, or green bodies. A green body is a body whose primary component is weakly bound powder material prior to melting and solidification. For example, a filler rod for welding may be composed of powdered material compressed into a rod suitable for use.

[0025] The particles may be solid, hollow, or a combination thereof. The particles may be fabricated by any means, such as, for example, gas atomization, milling, cryomilling, wire explosion, laser ablation, electrical discharge machining, or other techniques known in the art. The powder particles may be characterized by an average aspect ratio of about 1:1 to about 100:1. "Aspect ratio" refers to the ratio of the length to the width of a particle, expressed as length:width. A perfect sphere has an aspect ratio of 1:1. For particles of any shape, the length is considered to be the largest effective diameter, and the width is considered to be the smallest effective diameter. The particles may have one or more shapes, such as round, spherical, rod-like, crystalline, oval, etc. For example, some particles may be round, while others may be rod-like, and still others may have a crystalline shape. Various configurations are possible without departing from the spirit of this disclosure.

[0026] In some embodiments, the particles are rod-shaped. By "rod" is meant rod-like particles or domains shaped like long sticks, dowels, or needles. The average diameter of the rods can be selected, for example, from about 5 nanometers to about 100 microns. The rods need not be perfect cylinders, i.e., the axis is not necessarily straight and the diameter is not necessarily perfect circles. In the case of geometrically imperfect cylinders (i.e., when the axis is not exactly straight or the diameter is not exactly circular), the aspect ratio is the actual axial length along the line of curvature divided by the effective diameter, i.e., the diameter of a circle having the same area as the average cross-sectional area of ​​the actual nanorod shape. Particle size is measured using a Coulter Counter or known methods.

[0027] "Nanoparticle" refers to a particle having a maximum dimension between about 1 nm and about 10 microns. The preferred size of nanoparticles is less than about 250 nm, more preferably less than about 100 nm. As intended herein, "microparticle" refers to a particle having a maximum dimension between about 1 micron and about 100 microns. Nanoparticles or microparticles may be spherical or of any shape whose maximum dimension typically does not exceed the aforementioned maximum dimension. Exceptions are structures with extremely high aspect ratios, such as carbon nanotubes, which may have lengths up to about 100 microns but diameters less than about 100 nm. Nanoparticles or microparticles may include coatings of one or more layers of different materials. Mixtures of nanoparticles and microparticles may also be used. In some embodiments, the microparticles themselves are coated with nanoparticles, and the microparticle / nanoparticle composite is incorporated as a coating or layer on the base maraging steel alloy.

[0028] "Solidification" generally refers to a phase change from liquid to solid. In some embodiments, solidification refers to a phase change throughout the entire volume of the maraging steel. In other embodiments, solidification refers to a phase change at the surface of the maraging steel alloy or within a portion of the volume of the maraging steel alloy. In various embodiments, at least about 1%, 2%, 5%, 10%, 15%, 20%, 25%, 30%, 40%, 50%, 60%, 70%, 80%, 90%, 95%, 99%, or about 100% (by volume) of the maraging steel alloy melts and forms a liquid state. In certain embodiments, about 1% to about 90% (by volume) of the maraging steel alloy melts and forms a liquid state. In certain embodiments, about 2% to about 50% (by volume) of the functionalized maraging steel alloy melts and forms a liquid state. In certain embodiments, about 50% to about 100% (by volume) of the functionalized maraging steel alloy melts and forms a liquid state. During additive manufacturing, the maraging steel mixture can be melted and then solidified to form a maraging steel alloy having an equiaxed microstructure.

[0029] In the case of metals or metal mixtures, solidification generally results in one or more solid metallic phases, typically crystalline, but sometimes amorphous. Ceramics can also undergo crystalline or amorphous solidification. Metals and ceramics can form amorphous regions simultaneously with crystalline regions (e.g., in semicrystalline materials). In the case of certain polymers and glasses, solidification may not result in crystalline solidification. When an amorphous solid is formed from a liquid, solidification refers to the transition from a liquid above the glass transition temperature to an amorphous solid below the glass transition temperature. The glass transition temperature is not always clearly defined and may be characterized by a temperature range.

[0030] "Functionalization" or "surface functionalization" refers to a surface modification of a base maraging steel alloy, which modification significantly affects the solidification behavior (e.g., solidification rate, yield, selectivity, heat release, etc.) of the base maraging steel alloy. In various embodiments, the base maraging steel alloy is functionalized such that about 1%, 2%, 5%, 10%, 15%, 20%, 25%, 30%, 40%, 50%, 60%, 70%, 80%, 85%, 90%, 95%, 99%, or about 100% of the surface area of ​​the base maraging steel alloy has a surface functionalization modification. The surface modification may be a surface chemistry modification, a physical surface modification, or a combination thereof. In some embodiments, the functionalization is distinguishable by a detectable size difference between particles that differ by about one order of magnitude. For example, particles of about 1 cm can be functionalized with particles of about 100 μm.

[0031] In some embodiments, the surface functionalization comprises a grain growth inhibitor disposed along the base maraging steel alloy. The surface functionalization may comprise a grain growth inhibitor particle assembly chemically or physically disposed on the surface of the base maraging steel alloy.

[0032] Provided herein are maraging steel alloys, products fabricated from such alloys, and methods for producing such maraging steel alloys. Maraging steels are commonly used in applications requiring a high strength-to-weight ratio. Maraging steels have both high tensile strength and high fracture toughness, and typically do not require tempering.

[0033] Previous maraging steel alloys have been difficult to process using additive manufacturing. Additive manufacturing has traditionally been limited to weldable or castable alloys. Currently, there is only one high-strength steel alloy available for additive manufacturing, but this alloy suffers from anisotropic material properties and low fracture toughness due to its highly columnar grain structure. The large, columnar grains are highly oriented. Both the anisotropic material properties and low fracture toughness are typical of many additive steel alloys that tend to solidify in a columnar structure. Iron's crystalline structure generally has highly anisotropic elastic constants, which lead to poor overall material properties, including fatigue and fracture toughness. The highly anisotropic elastic constants, resulting from large differences in Schmid factors between differently oriented grains, lead to large local stresses and strains at grain boundaries under normal, moderate loads, causing premature failure and limiting the achievable strength and toughness. The alloy tends to fracture along the direction of the grain orientation.

[0034] Provided herein are novel maraging steel alloy systems that contain one or more grain growth inhibitors to generate unique microstructures during additive manufacturing. These grain growth inhibitors can enable the production of difficult-to-process alloys using additive manufacturing. The disclosed maraging steel alloys and methods for their production can generate wrought-like microstructures, rather than the columnar structures typically produced when used with additive manufacturing. In the present disclosure, additive manufacturing can be used to functionalize a base maraging steel alloy during processing to create a lattice match between the grain growth inhibitors and the base maraging steel alloy.

[0035] The grain growth inhibitors may be alloy-specific in composition and can be incorporated in high concentrations by incorporating the grain growth inhibitors during processing. The grain growth inhibitors can promote desirable equiaxed nucleation, resulting in an improved microstructure.

[0036] It has been shown that the use of certain grain growth inhibitors in additive processes with maraging steel alloys can enable the additive manufacturing of a variety of high-strength, but typically difficult-to-process, maraging steel alloys by reducing the propensity for cracking and creating an equiaxed microstructure that produces more isotropic material properties. It has been shown that higher concentrations of grain growth inhibitors can be used.

[0037] The present disclosure uses a base maraging steel alloy functionalized with a grain growth inhibitor, which can lattice match to a primary or secondary solidification phase in the base maraging steel alloy or react with an element in the base maraging steel alloy to form a phase that is lattice matched to a primary or secondary solidification phase in the base maraging steel alloy.

[0038] In some cases, mixtures of grain growth inhibitors may react with each other or with the base maraging steel alloy to form materials that are lattice matched to the primary or secondary solidification phase of the base maraging steel alloy.

[0039] Maraging steel alloys have unique crystal structures and melting points, making it difficult to incorporate additives into the alloy. Casting temperatures are generally very high in maraging steel alloys. Reaction kinetics at high temperatures make particle addition difficult due to dissolution or coarsening of the introduced particles. In the present disclosure, the melting temperature of the additives does not need to be very high, and the residence time can be very short (e.g., less than about 0.1 seconds), allowing for functionalization of the base maraging steel alloy. This is in contrast to the heating and holding at such temperatures (e.g., more than about 1 hour) required to cast maraging steel alloys.

[0040] The present maraging steel alloys can be made using any powder-based additive manufacturing process, such as selective laser melting (SLM), electron beam melting (EBM), laser engineered net shaping (LENS), and other powder-bed based processes. The base maraging steel alloy can be atomized to increase the surface area available for functionalization and then mixed with one or more grain growth inhibitors. In some embodiments, mixing in liquid form may not allow for uniform incorporation of the grain growth inhibitors, so the components can first be mixed in powder form.

[0041] During melting, incorporating a grain growth inhibitor into the melt can aid in the nucleation of new grains through lattice matching, reducing the critical amount of supercooling required for nucleation. In the absence of a grain growth inhibitor, the large temperature gradients encountered during additive manufacturing induce columnar growth. Columnar growth is a serious problem in the manufacturing of elastically anisotropic alloy systems. Forcing equiaxed growth by incorporating a grain growth inhibitor results in more isotropic material properties, thus strengthening the alloy system during the additive manufacturing process itself. Figures 10A-10C, discussed below, show images of various microstructures, including an equiaxed microstructure.

[0042] FIG. 1 is a schematic diagram of a functionalized maraging steel alloy 120 having either one type of grain growth inhibitor 110 or multiple types of grain growth inhibitors 110, 115 dispersed in a base maraging steel alloy 100. In the embodiment shown in FIG. 1, the functionalized maraging steel alloy 120 further includes a strengthening element 130, although the inclusion of the strengthening element 130 is optional. As shown in FIG. 1, in some embodiments, the base maraging steel alloy 100 is surface functionalized with the grain growth inhibitors 110, 115. That is, the grain growth inhibitors 110, 115 are dispersed along the surface 140 of the base maraging steel alloy 100. Methods for producing surface-functionalized maraging steel alloys, in some embodiments, are further described below.

[0043] The base maraging steel alloy 100 can include aluminum, cobalt, molybdenum, nickel, titanium, or a combination thereof, and in some embodiments can have a tensile strength greater than about 1000 MPa, alternatively greater than about 1300 MPa, or alternatively greater than about 1600 MPa.

[0044] In some embodiments, the grain growth inhibitors 110, 115 can include titanium, zirconium, boron, aluminum, tantalum, tungsten, carbon, niobium, cerium, or combinations thereof, as pure metals, oxides, hydrides, carbides, nitrides, intermetallic compounds, borides, or combinations thereof. In some embodiments, the grain growth inhibitors 110, 115 can include TiB, CeO, TiN, NbC, or combinations thereof. The grain growth inhibitors 110, 115 can comprise between about 0.01% and about 10% by volume of the functionalized maraging steel alloy 120, or in other embodiments, between about 0.01% and about 8%, between about 0.1% and about 5%, or between about 0.1% and about 1% by volume of the functionalized maraging steel alloy 120. The crystal growth inhibitors 110, 115 can form a nucleation phase and can be incorporated through functionalization in sufficiently high concentrations to reduce the critical supercooling required for equiaxed nucleation.

[0045] In some embodiments, the strengthening elements 130 may include nickel, aluminum, cobalt, chromium, molybdenum, carbon, manganese, niobium, zirconium, titanium, or combinations thereof.

[0046] As further described herein, the functionalized maraging steel alloy 120 can have an equiaxed microstructure. In some embodiments, the equiaxed microstructure can include a plurality of grains less than about 1 mm in diameter. In some embodiments, the equiaxed microstructure can include a uniform grain pattern along the x and y directions. In some embodiments, the equiaxed microstructure can form a scalloped pattern.

[0047] In some embodiments, the grain growth inhibitors 110, 115 may be nanoparticles. The small size of the nanoparticles and their reactivity may allow for the benefits demonstrated herein with a surface area coverage of less than 1%. In general, the grain growth inhibitors 110, 115 may have a chemical composition that differs from the chemical composition of the base maraging steel alloy 100.

[0048] In some embodiments, the grain growth inhibitors 110, 115 may be microparticles coating a micropowder or macropowder of the base maraging steel alloy 100. For other microparticles coating a micropowder or macropowder, functionalization may dictate that the grain growth inhibitors 110, 115 have significantly different dimensions than the base maraging steel alloy 100. For example, the grain growth inhibitors 110, 115 may be characterized by an average dimension (e.g., diameter) that is less than about 20%, 10%, 5%, 2%, or about 1% of the maximum dimension of the functionalized maraging steel alloy 120.

[0049] In some embodiments, surface functionalization also includes direct chemical or physical modification of the surface 140 of the base maraging steel alloy 100, for example, to enhance the bonding of the grain growth inhibitors 110, 115. Direct chemical modification of the surface 140 of the base maraging steel alloy 100, such as the addition of molecules, can also be used to affect the solidification behavior of the functionalized maraging steel alloy 120. Multiple surface modifications described herein can also be used simultaneously. The concentration of the grain growth inhibitors 110, 115 can be varied by changing the amount of functionalization at the surface 140 or by changing the concentration of functionalized maraging steel alloy relative to non-functionalized maraging steel alloy in the final feedstock.

[0050] Some variations provide a powdered base maraging steel alloy 100 comprising a plurality of particles, each having a particle surface area that is surface functionalized (e.g., continuously or intermittently) with a grain growth inhibitor 110, 115 selected to control solidification of the powdered base maraging steel alloy 100 from a liquid state to a solid state to form an equiaxed microstructure.

[0051] In some embodiments, the base maraging steel alloy is characterized in that, on average, at least about 1%, 2%, 5%, 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, or more, and up to about 100%, of the particle surface area is surface functionalized with a grain growth inhibitor 110, 115.

[0052] In some embodiments, the grain growth inhibitors 110, 115 are selected to control solidification of a portion of the base maraging steel alloy 100, such as a region of the powdered base maraging steel alloy 100 where solidification control is desired. There may also be other regions that include a conventional powdered base maraging steel alloy 100 that does not have the grain growth inhibitors 110, 115. In some embodiments, the grain growth inhibitors 110, 115 may be selected to control solidification of a portion of each particle (e.g., less than the entire volume of the particle, such as the outer shell).

[0053] A typical base maraging steel alloy 100 includes (but is not limited to) about 10 to about 25 wt.% nickel, 0 to about 20 wt.% cobalt, about 0.1 to about 15 wt.% molybdenum, about 0.01 to about 10 wt.% titanium, and about 0.01 to about 10 wt.% aluminum, with the remainder being iron and, optionally, carbon. For example, a typical base maraging steel alloy 100 includes about 15 to about 25 wt.% nickel, about 5 to about 15 wt.% cobalt, about 1 to about 10 wt.% molybdenum, about 0.1 to about 2 wt.% titanium, and about 0.01 to about 1 wt.% aluminum, with the remainder being iron and, optionally, carbon. For example, a typical base maraging steel alloy 100 contains about 17 to about 19 weight percent nickel, about 7 to about 12.5 weight percent cobalt, about 3 to about 5.2 weight percent molybdenum, about 0.15 to about 1.6 weight percent titanium, and about 0.05 to about 0.25 weight percent aluminum, with the remainder being iron and, optionally, carbon.

[0054] For example, a typical base maraging steel alloy 100 may contain about 17 to about 19 weight percent nickel, about 8 to about 9 weight percent cobalt, about 3 to about 3.5 weight percent molybdenum, about 0.15 to about 0.25 weight percent titanium, and about 0.05 to about 0.15 weight percent aluminum, with the balance being iron and, optionally, carbon. For example, a typical base maraging steel alloy 100 may contain about 17 to about 19 weight percent nickel, about 7 to about 8.5 weight percent cobalt, about 4.6 to about 5.2 weight percent molybdenum, about 0.3 to about 0.5 weight percent titanium, and about 0.05 to about 0.15 weight percent aluminum, with the balance being iron and, optionally, carbon. For example, a typical base maraging steel alloy 100 may contain about 18 to about 19 weight percent nickel, about 8.5 to about 9.5 weight percent cobalt, about 4.6 to about 5.2 weight percent molybdenum, about 0.5 to about 0.8 weight percent titanium, and about 0.05 to about 0.15 weight percent aluminum, with the balance being iron and, optionally, carbon. For example, a typical base maraging steel alloy 100 may contain about 18 to about 19 weight percent nickel, about 11.5 to about 12.5 weight percent cobalt, about 4.6 to about 5.2 weight percent molybdenum, about 1.3 to about 1.6 weight percent titanium, and about 0.05 to about 0.15 weight percent aluminum, with the balance being iron and, optionally, carbon.

[0055] In some embodiments, base maraging steel alloy 100 can include manganese to replace nickel or reduce the amount of nickel in the alloy. For example, base maraging steel alloy 100 can include about 9 to about 15 wt.% manganese. In some embodiments, base maraging steel alloy 100 can be a cobalt-free maraging steel. For example, base maraging steel alloy 100 can include about 18.9 wt.% nickel, 4.1 wt.% molybdenum, and 1.9 wt.% titanium, with the balance being iron.

[0056] The selection of the base maraging steel alloy 100 and the grain growth inhibitors 110, 115 can depend on the desired properties and should be considered on a case-by-case basis. The processing and final product configuration can also depend on the desired properties.

[0057] 13 illustrates a method of manufacturing the functionalized maraging steel alloy 120. As shown in FIG. 13, in some embodiments, the method of manufacturing the maraging steel alloy can include step 200 of mixing a base maraging steel alloy with a grain growth inhibitor to provide a maraging steel mixture, step 210 of melting the maraging steel mixture, and step 215 of solidifying the maraging steel mixture to form an equiaxed microstructure. In some embodiments, the method can include step 230 of melting a base maraging steel alloy, step 235 of mixing a grain growth inhibitor into the melt of the base maraging steel alloy to form a maraging steel mixture, and then step 215 of solidifying the maraging steel mixture to form an equiaxed microstructure.

[0058] 13, solidifying the maraging steel mixture to form an equiaxed microstructure 215 can include solidifying a first layer of the maraging steel mixture along a single axis and then solidifying an adjacent layer of the maraging steel mixture along the same axis 220. Optional steps are illustrated by dashed arrows.

[0059] In some embodiments, the base maraging steel alloy 100 can exist as a powder when mixed with the grain growth inhibitors 110, 115, such as in step 200, where the base maraging steel alloy is mixed with the grain growth inhibitors to provide the maraging steel mixture.

[0060] In some embodiments, the base maraging steel alloy 100 and the grain growth inhibitors 110, 115 have a lattice strain of less than about 10%, for example, less than about 8%, less than about 5%, or less than about 10%, such as from about 0.001 to about 10%, from about 0.01 to about 8%, or from about 0.1 to about 5%. Lattice strain can be measured by known methods, such as measuring microscopic lengths using XRD, measuring strain and unstrained strain delta d, and calculating using known methods. In some embodiments, the base maraging steel alloy 100 and the grain growth inhibitors 110, 115 can have an atomic density difference of less than about 35%, for example, less than about 30%, less than about 25%, or less than about 35%, such as from about 0.001 to about 35%, from about 0.01 to about 30%, or from about 0.1 to about 25%. Atomic density can be measured by known methods, such as measuring microscopic lengths using XRD and calculating using known methods. The atomic density and lattice distortion can be calculated from standard crystal structures. The atomic density difference is small and therefore energetically favorable. For example, a large atomic density difference may result in extra or missing atoms that have nowhere to align, resulting in an energetically unfavorable configuration.

[0061] In some embodiments, the base maraging steel alloy 100 may further include strengthening elements 130 including nickel, aluminum, cobalt, chromium, molybdenum, carbon, manganese, niobium, zirconium, titanium, or combinations thereof.

[0062] In some embodiments, the step of solidifying the maraging steel mixture may include semi-passively controlling the solidification of the maraging steel mixture within the initially powdered material from a liquid state to a solid state.

[0063] As intended herein, terms such as "semi-passive control," "semi-passively controlled," and the like refer to control of the heating, cooling, and solidification of the functionalized maraging steel alloy 120 during both heating and cooling, which is engineered prior to melting by the selected functionalization and is not actively controlled externally once the melting-solidification process begins. It should be noted that external interactions are not necessarily avoided. In some embodiments, semi-passive control of solidification further includes selecting the atmosphere (e.g., pressure, humidity, or gas composition), temperature, or heat input or output. These factors, and others known to those skilled in the art, may or may not be included in the semi-passive control. The incorporation of grain growth inhibitors 110, 115 allows for semi-passive control of the maraging steel mixture to achieve an equiaxed microstructure 185 in the final solid product.

[0064] Next, we describe a typical semi-passive control process enabled by the surface functionalization described herein.

[0065] One route to controlling nucleation is to introduce a grain growth inhibitor 110, 115 into the base maraging steel alloy 100 to create a maraging steel mixture. The grain growth inhibitor 110, 115 can comprise any of the material compositions described above and can be selected based on the melt wettability of the base maraging steel alloy 100. At the beginning of melting, the grain growth inhibitor 110, 115 wets into the melt pool as dispersed particles that act as nucleation sites upon cooling, creating a fine-grained structure with observable nucleation sites in cross section. In some embodiments, the density of nucleation sites is increased, thus increasing the volumetric solidification rate due to the number of growing solidification fronts and the lack of a nucleation energy barrier.

[0066] In an exemplary embodiment, the grain growth inhibitors 110, 115 can be introduced into a melt pool of the base maraging steel alloy 100 in an additive manufacturing process. The grain growth inhibitors 110, 115 then disperse in the melt pool and act as nucleation sites for the solid. The additional, well-dispersed nucleation sites can mitigate shrinkage cracking (hot cracking). Shrinkage cracking typically occurs when liquid cannot reach certain areas due to blockage of narrow channels between solidifying particles. Increasing the number of nucleation sites can prevent the formation of long, narrow channels between solidifying particles, as many small particles grow instead of a few large particles.

[0067] In another exemplary embodiment, the grain growth inhibitors 110, 115 can function as nucleation sites for secondary phases in the base maraging steel alloy 100. The grain growth inhibitors 110, 115 can include a secondary phase or a material that nucleates the secondary phase (e.g., by a similar crystal structure). This embodiment can be beneficial when the secondary phase is responsible for blocking interdendritic channels that lead to hot cracking. Nucleation of multiple small particles of the secondary phase can avoid large particles that can block narrow interdendritic channels. Furthermore, this embodiment can be beneficial when the secondary phase has a tendency to form a continuous phase between primary phase particles that promotes stress corrosion cracking. By providing additional nucleation sites for the secondary phase, the secondary phase can be decomposed and dispersed without forming a continuous phase between primary alloy particles. Decomposing the secondary phase during solidification offers the potential for more complete homogenization of the material during heat treatment, thereby reducing the likelihood of stress corrosion cracking (lower gradients in homogenized materials). If the secondary phase is not continuous, it is less likely to form long corrosion notches.

[0068] In another embodiment of nucleation control, the functionalized surface 140 of the base maraging steel alloy 100 can be fully or partially dissolved in the melt and react with materials in the melt to form precipitates or inclusions that can function in the same manner as the grain growth inhibitors 110, 115 in the previous paragraph.

[0069] In another embodiment, the grain growth inhibitors 110, 115 can react with impurities to form nucleation sites. For example, carbon and oxygen can be present as impurities in maraging steels and can react with the grain growth inhibitors 110, 115.

[0070] 2A is a schematic diagram of semi-passive solidification control including nucleation control. Grain growth inhibitors 110 are placed between base maraging steel alloy 100 to form maraging steel mixture 132. The grain growth inhibitors 110 act as nucleation sites, resulting in equiaxed grains 135 that form equiaxed microstructure 185 in the final solid material.

[0071] 2B is a schematic diagram of semi-passive solidification control including nucleation control. Grain growth inhibitors 110 are placed between the base maraging steel alloy 100 to form the maraging steel mixture 132. The grain growth inhibitors 110 prevent the unstoppable growth of individual dendrites, resulting in equiaxed grains 135 that form an equiaxed microstructure 185 in the final solid maraging steel alloy.

[0072] FIG. 2C is a schematic diagram of an exemplary additive manufacturing process for functionalized maraging steel alloy 120. Grain growth inhibitors 110 are disposed between base maraging steel alloy 100 to form maraging steel mixture 132. Maraging steel mixture 132 is melted to form melt 145, which is then solidified layer by layer to form the final solid product. For example, as shown in FIG. 2C, a first layer 146 is solidified, followed by a second layer 147 adjacent to and positioned along the same axis as first layer 146. Various techniques can be used to form the final solid product, such as selective laser melting (SLM), electron beam melting (EBM), laser direct deposition (LENS), or a combination thereof.

[0073] The grain growth inhibitors 110, 115 can promote surface growth of crystals with good epitaxial match. Nucleation at the surface of the grain growth inhibitors 110, 115 can be more likely when there is a good match between the crystal lattice parameters of the grain growth inhibitors 110, 115 and the solidifying base maraging steel alloy 100. The grain growth inhibitors 110, 115 can be selected to promote nucleation of specific phases in the melt.

[0074] In general, the chemical reaction that promotes nucleation may depend on the selected surface functionalization and heating (or cooling) parameters.

[0075] Because the grain growth inhibitors 110, 115 are organized on the surface 140 of the base maraging steel alloy 100 under conditions where melting or near-melting occurs rapidly and rapidly fuses particles together with little melt convection, the grain growth inhibitors 110, 115 do not have the time or associated energy to diffuse away from their initial location relative to the other powders. This can create a three-dimensional network of inclusions. This provides a method for controlling maximum grain size and / or designing a predictable microstructure. The microstructure can depend on the initial powder size, shape, and packing configuration / density. Adjusting coating and powder parameters can enable control of this hierarchical structure. In some embodiments, these architectures can significantly improve material properties by impeding, blocking, or redirecting dislocation motion in specific directions, thereby reducing or eliminating failure mechanisms.

[0076] Using appropriate functionalization, heat flow during solidification can be controlled using appropriate heats of fusion or vaporization. In some embodiments, inclusions can be drawn into the melt (as described above) or reacted within the melt. In some embodiments, grain growth inhibitors 110, 115 can be repelled to the surface of the melt pool. Utilizing a functionalized surface with a high vapor pressure at the desired melting point of the powder can provide a cooling effect on the melt, causing vaporization and increasing the solidification rate. This effect can be easily detected when comparing a non-functionalized maraging steel alloy to a functionalized maraging steel alloy 120 under identical conditions, and also when comparing the composition of the raw materials to the composition of the final product.

[0077] In other embodiments, the opposite effect may occur. Some systems may require slower solidification times than can reasonably be provided in a particular production system. In this case, a high-melting material, which may be rejected to the surface, solidifies. This releases the heat of fusion into the system, slowing the overall heat flux from the melt. It is also possible to retain heat in the melt and slow solidification by incorporating a secondary material with a significantly higher heat capacity.

[0078] In another embodiment, the heat of formation can be used to control heat flow during melt pool formation and / or solidification. For example, certain grain growth inhibitors 110, 115 can be added to the base maraging steel alloy 100, which, upon supply of sufficient activation energy, can cause an exothermic reaction of the components. This can release a large heat of formation that can aid in the complete or partial melting of the particles. The resulting grain growth inhibitors 110, 115 can be absorbed into the melt and, due to their high melting points, remain suspended as solids (even if partially dissolved), thereby acting as nucleation sites and strengthening the subsequent alloy.

[0079] Thermodynamic control of solidification can utilize grain growth inhibitors 110, 115 that cause phase transformations that are different from those in the base maraging steel alloy. The phase transformations can occur at different solidus and / or liquidus temperatures, similar solidus and / or liquidus temperatures, or the same solidus and / or liquidus temperatures. The grain growth inhibitors 110, 115 after the phase transformation can be incorporated into the final solid material, excluded from the final solid material, or both. The grain growth inhibitors 110, 115 after the phase transformation can be miscible or immiscible with the molten state. The grain growth inhibitors 110, 115 after the phase transformation can be miscible or immiscible with the solid state.

[0080] Thermodynamic control of solidification can utilize vaporized or partially vaporized crystal growth inhibitors 110, 115. For example, such coatings can include organic materials (e.g., waxes, carboxylic acids, etc.) or inorganic salts (e.g., MgBr2, ZnBr2, etc.).

[0081] Thermodynamic control of solidification can utilize crystal growth inhibitors 110, 115 that release or absorb gases (eg, oxygen, hydrogen, carbon dioxide, etc.).

[0082] Thermodynamic control of solidification can utilize grain growth inhibitors 110, 115 that have a different heat capacity than the base maraging steel alloy 100.

[0083] In addition to controlling the energy in the system, it is also possible to control the rate of heat release from the system by controlling the thermal conductivity or thermal emissivity (thermal IR radiation). This type of control can be derived, for example, from the ejection to the surface or the thermal conductivity of a powder bed during additive manufacturing. In one embodiment, functionalization can eject a low-conductivity material, which can be the functionalization material directly or its reaction product, to the surface, thus insulating the melt below and slowing the solidification rate. In other embodiments, layers can have high / low emissivity, which would increase / decrease the radiative heat flow into / out of the system. These embodiments are particularly applicable to electron beam systems that are under vacuum and therefore where radiation is the primary mechanism for heat flow.

[0084] 5A is a schematic diagram of semi-passive solidification control involving thermodynamic control in which crystal growth inhibitors 110 react within the melt 145 and the reaction enthalpy is utilized to control heat flow during solidification. After the crystal growth inhibitors 110 react within the melt 145, along with the control of heat flow, new crystal growth inhibitors 115 can be generated upon solidification into a solid material 150.

[0085] 5B is a schematic diagram of semi-passive solidification control involving thermodynamic control in which a crystal growth inhibitor 110 or its reaction product is driven to the surface of the melt 145, where it vaporizes and removes heat from the solidified material 150. The crystal growth inhibitor may be present at the surface, for example, as a crystal growth inhibitor layer 155.

[0086] In another embodiment, the functionalized surface 140 may be fully absorbed into the melt, but proximity to other unmelted functionalized maraging steel alloys 120, such as in powder bed additive manufacturing, may alter the heat transfer from the system. This may manifest as a low thermal conductivity base powder with a high conductivity coating.

[0087] FIG. 6A is a schematic diagram of semi-passive solidification control including conductivity or emissivity control in which crystal growth inhibitor 110 driven to the surface of melt 145 forms crystal growth inhibitor layer 155 having a different conductivity or emissivity than the underlying solidified material 150.

[0088] FIG. 6B is a schematic diagram of semi-passive solidification control in which the crystal growth inhibitor 110 remains distributed in the melt 145 and includes conductivity or emissivity control to change the conductivity of the melt 145 and the final solid material 150.

[0089] Thermal conductivity or emissivity control of solidification can utilize grain growth inhibitors 110, 115, which have higher thermal conductivity compared to the base maraging steel alloy 100. The grain growth inhibitors 110, 115 can be incorporated into the melt, or can be rejected, such as at grain boundaries, or can be rejected, such as at the surface of the melt as a grain growth inhibitor layer 155. The grain growth inhibitors 110, 115 can be miscible or immiscible with the molten state. The grain growth inhibitors 110, 115 can be miscible or immiscible with the final solid material 150.

[0090] Thermal conductivity or emissivity control of solidification can utilize grain growth inhibitors 110, 115 that have lower thermal conductivity compared to the base maraging steel alloy 100.

[0091] Thermal conductivity or emissivity control of solidification can utilize grain growth inhibitors 110, 115 that have higher emissivity compared to the base maraging steel alloy 100.

[0092] Thermal conductivity or emissivity control of solidification can utilize grain growth inhibitors 110, 115 that have low emissivity compared to the base maraging steel alloy 100.

[0093] In some embodiments, the functionalized maraging steel alloy 120 can react with contaminants in the melt. When the functionalized maraging steel alloy 120 is properly selected, the resulting material can be selected so that the reaction products formed have high surface tension with the liquid and are therefore rejected to the surface. The rejected reaction products can take the form of easily removable scale. Optionally, the rejected layer is not actually removed but rather incorporated into the final product. The rejected layer can appear as a hard-faced carbide, nitride, or oxide coating, a soft, galling-resistant material, or any other functional surface that can improve the desired properties of the produced material. In some cases, the rejected surface layer can be a composition and can be subjected to a cooling regime that can result in an amorphous layer on the surface of the material after solidification. These surface-rejected structures can result in improved properties related to, but not limited to, improved corrosion resistance, stress corrosion cracking resistance, crack initiation resistance, overall strength, wear resistance, emissivity, reflectivity, and magnetic susceptibility.

[0094] 7A is a schematic diagram of semi-passive solidification control including contaminant removal and exclusion to the surface. In the embodiment shown in FIG. 7A, a base maraging steel alloy 100 has grain growth inhibitors 110 disposed on a surface 140 of the base maraging steel alloy 100. The grain growth inhibitors 110 are distributed in a melt 145 and react with contaminants (not shown) from the melt 145 to form new grain growth inhibitors 115. The reacted contaminants can be excluded to the surface of the final solid material 150, for example, as a contaminant surface layer 160.

[0095] 7B is a schematic diagram of semi-passive solidification control that involves reaction with contaminants, which remain in the solid. The crystal growth inhibitors 110 distribute in the melt 145 and react with contaminants (not shown) from the melt 145 to form new crystal growth inhibitors 115. The new crystal growth inhibitors 115 can remain in the final solid material 150.

[0096] Several scenarios are possible through the removal or elimination of contaminants. A crystal growth inhibitor 110, 115 that reacts or binds with undesired contaminants can be incorporated into the solidification in the same phase or in a separate solid phase. The reacted crystal growth inhibitor 110, 115 can be rejected upon solidification. If a moiety or some element present in the crystal growth inhibitor 110, 115 reacts or binds with the contaminant, such moiety or element can be incorporated and / or rejected.

[0097] In some embodiments, the grain growth inhibitors 110, 115 can react upon heating, such as by a eutectic reaction, to form a material with a lower melting point compared to the base maraging steel alloy 100. The grain growth inhibitors 110, 115 can be selected from materials that react with the underlying base maraging steel alloy 100 to initiate melting at the surface 140 or at a partial volume of the base maraging steel alloy 100. A heat source, such as a laser or electron beam, can be selected so that the energy density is high enough to initiate a surface reaction but not completely melt the entire base maraging steel alloy 100. This causes uniform liquid phase sintering at the grain surfaces. After solidification, the structure has a distinctive microstructure exhibiting a variable composition and grain nucleation pattern around a central core of the raw powder that has a similar microstructure to the raw powder after undergoing a similar heat treatment. This structure can then be normalized or post-treated to increase density or improve properties.

[0098] Another possible reaction is a peritectic reaction in which one grain growth inhibitor 110, 115 melts and this molten material diffuses into the second grain growth inhibitor 110, 115 to form an alloyed solid. This new alloyed solid can then act as a center for phase nucleation or can limit melting just at the grain edges.

[0099] Figure 3 is a schematic diagram of semi-passive solidification control involving a peritectic reaction. In the embodiment shown in Figure 3, a base maraging steel alloy 100 has grain growth inhibitors 110 disposed on the surface of the base maraging steel alloy 100. In Figure 3, the grain growth inhibitors 110 are distributed in a melt 145. At high temperatures, the grain growth inhibitors 110 melt, forming a melt 170. Upon cooling, a peritectic reaction occurs, and the grain growth inhibitors form dispersoids 175 in the melt 180.

[0100] Introducing the grain growth inhibitors 110, 115 into the molten metal can be difficult if the grain growth inhibitors 110, 115 have a thin oxide layer on their surface because the liquid metal typically does not wet the oxide well. This can cause the grain growth inhibitors 110, 115 to be pushed to the surface of the melt 145. One way to overcome the oxide layer on the grain growth inhibitors 110, 115 and the associated wetting issues is to form the grain growth inhibitors 110, 115 in situ during the formation of the melt pool. This can be achieved by starting with an elemental grain growth inhibitor 110, 115 that forms an intermetallic compound with one component of the base maraging steel alloy 100, while avoiding decomposition of the grain growth inhibitors 110, 115 in the melt 145. Alternatively, binary compound crystal growth inhibitors 110, 115 that dissociate at high temperatures, such as hydrides or nitrides, can be used because the dissociation reaction destroys any oxide shell that may be present on the crystal growth inhibitors 110, 115.

[0101] As discussed above, surface functionalization can be designed to react and be rejected to the surface of the melt 145. In embodiments employing additive manufacturing, layered structures can be designed. In some embodiments, progressive build layers and hatching can be heated such that each successive melt 145 is heated long enough to reject the subsequent rejected layer, resulting in a build with an exterior scale and little or no layer of rejected material observed within the build. In other embodiments, particularly those in which functional or desired materials are rejected to the surface, heating and hatching procedures can be used to produce composite structures with layered end products. Depending on the build parameters, these can be layered structures of random orientation and design that can be used to produce materials with significantly improved properties.

[0102] Figure 4 is a schematic diagram of semi-passive solidification control. In the embodiment shown in Figure 4, a base maraging steel alloy 100 has grain growth inhibitors 110 disposed on a surface 140 of the base maraging steel alloy 100. As shown in Figure 4, the melt 145 solidifies the collection of grain growth inhibitors 110 without significant movement, allowing the grain growth inhibitors 110 to orient into a three-dimensional structure (multiple grain growth inhibitors 110 in solidified material 460) that is repeated throughout the final solid material 150.

[0103] Engineered microstructures can be designed such that feature size within the three-dimensional network (e.g., distance between nodes of crystal growth inhibitors) is selected along with a target composition for an intended purpose. Similarly, layered composite structures can be designed such that feature size (e.g., layer thickness or distance between layers) is selected along with a target composition for an intended purpose.

[0104] It should be noted that surface exclusion is not necessary to produce a layered structure. The functionalized surfaces can be relatively immobile from their initial location on the surface 140 of the base maraging steel alloy 100. Upon melting, these functionalized surfaces can act as nucleation sites as described above, but instead of being absorbed into the melt, they can initiate nucleation in unmelted locations previously occupied by the powder surface. As a result, a fine-grained structure develops from the surface nucleation sources toward the center. This can result in an engineered composite structure with improved properties over the base maraging steel alloy 100. In general, this mechanism allows for the control of desired incorporation locations through controlled solidification.

[0105] In additive manufacturing of maraging steel alloy 120, the issue of microstructural texturing of subsequent layers of molten metal results in an anisotropic microstructure and therefore anisotropic structural properties. By dispersing stable grain growth inhibitors 110, 115 in the solidified layer, equiaxed grains 135 with stable isotropic morphology can be produced over repeated heating cycles.

[0106] Any solidification control method that derives its primary function from the surface functionalization of a powdered material is considered within the scope of the present invention. Other control methods can include multiple types of control as described above. Examples of combinations of methods include the use of surface rejection, internal reactions, and emissivity control. For example, a part can be processed using additive manufacturing, where a functionalization material is selected to melt to the surface and reacts to form an insoluble material that is rejected to the surface of the melt pool. This rejected material can then have a low emissivity that reflects additional laser radiation, reducing local heating and allowing the material to cool more quickly and control solidification. The resulting structure is a controlled solidification structure material with a low-emissivity surface coating.

[0107] In some embodiments, the solid state is a three-dimensional microstructure that includes the crystal growth inhibitors 110, 115 as inclusions distributed throughout the solid state.

[0108] In some embodiments, the solid state is a layered microstructure that includes one or more layers that contain the crystal growth inhibitors 110, 115.

[0109] The method may further include creating the structure by one or more techniques selected from the group consisting of additive manufacturing, injection molding, pressing and sintering, capacitive discharge sintering, and spark plasma sintering. The invention may provide a solid object or article comprising a structure produced using such a method.

[0110] FIG. 8 is a schematic illustration of surface melting of a functionalized maraging steel alloy 120 where heat is applied and the grain growth inhibitor 110 reacts with the surface 140 of the base maraging steel alloy 100 to form a melt in a coating 180 that covers less than 100% of the surface 140.

[0111] In some embodiments, the surface functionalization is in the form of a continuous or intermittent coating 180. A continuous coating 180 can cover at least 90% of the surface 140, such as about 95%, about 99%, or about 100% of the surface 140 (recognizing the possibility of defects, voids, or impurities in the surface). An intermittent coating 180 can be non-continuous and cover less than about 90%, such as about 80%, about 70%, about 60%, about 50%, about 40%, about 30%, about 20%, about 10%, about 5%, about 2%, about 1%, or less of the surface 140. The intermittent coating 180 can be uniform (e.g., having a particular repeating pattern on the surface 140) or non-uniform (e.g., random).

[0112] In general, coating 180 can be continuous or discontinuous. Coating 180 can have several characteristic morphologies. In one embodiment, coating 180 can be smooth and conform to the underlying surface 140. In another embodiment, coating 180 can be nodular. Nodular growth is characteristic of kinetic limits of nucleation and growth. For example, coating 180 can appear like a cauliflower or small fractals growing from surface 140. These morphologies can be affected by the underlying material, coating method, reaction conditions, etc.

[0113] Coating 180 may or may not be in the form of nanoparticles or microparticles. That is, coating 180 may be derived from nanoparticles or microparticles, but distinct nanoparticles or microparticles may no longer be present. Various coating techniques may be used, such as, but not limited to, electroless deposition, dip deposition, or solution coating. The thickness of coating 180 is preferably less than about 20% of the diameter of the underlying particle, such as less than about 15%, 10%, 5%, 2%, or about 1% of the diameter of the underlying particle.

[0114] Some variations provide structures made from the functionalized maraging steel alloy 120 by additive manufacturing. The functionalized maraging steel alloy 120 (with the grain growth inhibitors 110, 115) can be incorporated into the final structure. In some embodiments, the grain growth inhibitors 110, 115 are rejected, forming a scale. This scale may not be bonded to the structure. In some embodiments, the scale is bonded to the structure or is otherwise not easily removable. This can advantageously provide structural reinforcement, for example, the rejected ceramic particles can add a hard surface to the final structure. The rejected grain growth inhibitors 110, 115 can form multilayer composites, with each layer having a different composition. In some embodiments, the rejected grain growth inhibitors 110, 115 form spatially varying compositions within the overall structure. Three-dimensional structures can also be developed within the final microstructure.

[0115] 9 is a schematic illustration of the formation of a layered composite structure in which a functionalized maraging steel alloy 120 with two different types of grain growth inhibitors 110, 115 results in different grain separations, resulting in a layered structure having a first grain growth inhibitor layer 181, a base maraging steel alloy layer 182, and a second grain growth inhibitor layer 183. In the schematic illustration of FIG. 9, the first grain growth inhibitor layer 181 results from the grain growth inhibitor 110 (or its reaction), the base maraging steel alloy layer 182 results from the base maraging steel alloy 100 (or its reaction), and the second grain growth inhibitor layer 183 results from the grain growth inhibitor 115 (or its reaction).

[0116] Some variations provide a final solid material 150 or article that (i) includes the powdered base maraging steel alloy 100 described above, or (ii) includes at least one solid phase obtained from the liquid form of the base maraging steel alloy 100 described above. The solid phase can form from about 0.25% to about 100% by weight of the final solid material 150 or article, such as, for example, about 1%, 5%, 10%, 25%, 50%, or about 75% by weight of the final solid material 150 or article.

[0117] Another variation of the present invention provides a final solid material 150 or article comprising a continuous solid phase and a three-dimensional network of inclusions of crystal growth inhibitors 110, 115 distributed throughout the continuous solid phase, wherein the three-dimensional network blocks, impedes, or redirects dislocation motion in the final solid material 150 or article.

[0118] The crystal growth inhibitors 110, 115 can have an average maximum particle size of about 1 nanometer to about 100 microns. In some embodiments, the average maximum particle size can be less than about 100 nanometers. In these or other embodiments, the crystal growth inhibitors 110, 115 can have an average minimum particle size of about 1 nanometer to about 1 micron, such as less than about 100 nanometers. "Average maximum particle size" refers to the number average of the maximum particle size for all crystal growth inhibitors 110, 115 present. "Average minimum particle size" refers to the number average of the minimum particle size for all crystal growth inhibitors 110, 115 present. A perfect sphere has only one dimension, the diameter, which is both the minimum particle size and the maximum particle size. A cylinder has two characteristic length measures: length (height) and diameter. If the cylinder is in the form of a long rod, the maximum particle size dimension is the length and the minimum particle size dimension is the diameter. In various embodiments, the crystal growth inhibitor can have an average maximum particle dimension of about 10, 25, 50, 75, 100, 150, 200, 250, 300, 400, 500, 600, 700, 800, 900, or about 1000 nanometers, or less than about 10, 25, 50, 75, 100, 150, 200, 250, 300, 400, 500, 600, 700, 800, 900, or about 1000 nanometers. In various embodiments, the crystal growth inhibitors 110, 115 can have an average minimum particle size of about 5, 10, 25, 50, 75, 100, 150, 200, 250, 300, 400, or about 500 nanometers, or less than about 5, 10, 25, 50, 75, 100, 150, 200, 250, 300, 400, or about 500 nanometers.

[0119] In some embodiments, light elements are incorporated into the system. For example, the surface of the base maraging steel alloy 100 can be reacted with an element selected from the group consisting of hydrogen, oxygen, carbon, nitrogen, boron, sulfur, and combinations thereof. For example, a reaction with hydrogen gas can be carried out to form a metal hydride. Optionally, the base maraging steel alloy 100 or the grain growth inhibitor 110, 115 can further include salts, carbon, organic additives, inorganic additives, or combinations thereof. Certain embodiments utilize relatively inert carbides that are incorporated (e.g., into the steel) upon rapid melting and solidification.

[0120] Methods for producing the surface-functionalized maraging steel alloy 120 are generally not limited to and can include immersion deposition, electroless deposition, vapor coating, solution / suspension coating of particles with or without organic ligands, utilizing electrostatic and / or van der Waals forces to adhere particles through mixing, etc. U.S. Patent Application No. 14 / 720,757 (filed May 23, 2015), U.S. Patent Application No. 14 / 720,756 (filed May 23, 2015), and U.S. Patent Application No. 14 / 860,332 (filed September 21, 2015) are each commonly owned with the assignee of the present patent application. These disclosures, in some embodiments, relate to methods for coating certain materials onto micropowder.

[0121] For example, as described in U.S. Patent Application No. 14 / 860,332, the crystal growth inhibitors 110, 115 can be applied using immersion deposition in an ionic liquid, which deposits a more noble metal onto a less noble, more electronegative metal substrate by chemical displacement from a solution of the coating metal's metal salt. This method does not require an external electric field or additional reducing agent, as is required in standard electroplating or electroless plating, respectively. The metal can be selected from the group consisting of aluminum, zirconium, titanium, zinc, nickel, cobalt, copper, silver, gold, palladium, platinum, rhodium, titanium, molybdenum, uranium, niobium, tungsten, tin, lead, tantalum, chromium, iron, indium, rhenium, ruthenium, osmium, iridium, and combinations or alloys thereof.

[0122] In some embodiments, an organic ligand can be reacted to the metal. The organic ligand can be selected from the group consisting of aldehydes, alkanes, alkenes, silicones, polyols, poly(acrylic acid), poly(quaternary ammonium salts), poly(alkylamines), poly(alkylcarboxylic acids) including copolymers of maleic anhydride or itaconic acid, poly(ethyleneimine), poly(propyleneimine), poly(vinylimidazolines), poly(trialkylvinylbenzylammonium salts), poly(carboxymethylcellulose), poly(D- or L-lysine), poly(L-glutamic acid), poly(L-aspartic acid), poly(glutamic acid), heparin, dextran sulfate, 1-carrageenan, pentosan polysulfate, mannan sulfate, chondroitin sulfate, and combinations or derivatives thereof.

[0123] The reactive metal can be selected from the group consisting of alkali metals, alkaline earth metals, aluminum, silicon, titanium, zirconium, hafnium, zinc, and combinations or alloys thereof, hi some embodiments, the reactive metal is selected from aluminum, magnesium, or alloys containing greater than 50 atomic percent aluminum and / or magnesium.

[0124] The final solid material 150 may have a porosity of 0% to about 75%, such as about 5%, 10%, 20%, 30%, 40%, 50%, 60%, or 70%, in various embodiments. Porosity may come from both intraparticle voids (e.g., hollow shapes) and voids outside and between particles. Overall porosity takes into account porosity from both sources.

[0125] In some embodiments of the present invention, particles of the functionalized maraging steel alloy 120 can be fused together to form a continuous or semi-continuous material. As intended herein, "fused" should be broadly interpreted to mean any manner in which particles are at least partially bonded, joined, coalesced, or otherwise united together. Many known techniques can be used to fuse particles together.

[0126] In various embodiments, fusion is achieved by sintering, heat treatment, pressure treatment, combined heat / pressure treatment, electrical treatment, electromagnetic treatment, melting / solidification, contact (low temperature) welding, solution combustion synthesis, self-propagating high temperature synthesis, solid state metathesis, or a combination thereof.

[0127] When thermal treatment is utilized, heat or energy can be provided by electrical current, electromagnetic energy, chemical reactions (including ionic or covalent bond formation), electrochemical reactions, pressure, or a combination thereof. Heat can be provided to initiate chemical reactions (e.g., overcome activation energy), enhance reaction rates, shift reaction equilibria, or adjust reaction network distributions.

[0128] Some possible powder metallurgical processing techniques that can be used include, but are not limited to, hot pressing, sintering, high pressure low temperature sintering, extrusion, metal injection molding, and additive manufacturing.

[0129] The final solid material 150 can be produced by a process selected from the group consisting of hot pressing, cold pressing and sintering, extrusion, injection molding, additive manufacturing, electron beam melting, selective laser sintering, pressureless sintering, and combinations thereof. The solid article can be, for example, a coating, a coating precursor, a substrate, a billet, a net shape part, a near net shape part, or other object.

[0130] The present disclosure is applicable to additive manufacturing and welding applications, as well as many other applications. Some embodiments provide powder metallurgical processed parts that are equivalent to machined parts. Some embodiments provide corrosion resistant surface coatings that are formed during the manufacture of the part, rather than as an additional step.

[0131] Other commercial applications include, but are not limited to, complex component consolidation (reducing the number of individual parts used to make one assembly), lightweight and optimized structures, battery and fuel cell electrodes, catalytic materials, lightweight fillers, complex tooling, and improving the performance of existing parts.

[0132] In some embodiments, the concentration of the grain growth inhibitors may depend on the selected base maraging steel alloy 100 and the grain growth inhibitors 110, 115. In some embodiments, the amount of the grain growth inhibitors 110, 115 may be greater than about 0.01% by volume of the functionalized maraging steel alloy 120, and in some embodiments, the amount of the grain growth inhibitors 110, 115 may be greater than about 0.05% by volume of the functionalized maraging steel alloy 120. In some embodiments, the amount of the grain growth inhibitors 110, 115 may be greater than about 0.5% by volume, or greater than about 1% by volume of the functionalized maraging steel alloy 120.

[0133] Grain size can be measured using ASTM E112, "Standard Test Methods for Determining Average Grain Size," to demonstrate the grain growth inhibition potential of the present disclosure. For example, in some cases, grain growth inhibitors may be required at greater than about 0.5 volume percent, or even greater than about 1 volume percent, of the functionalized maraging steel alloy. In certain alloys, minimizing grain growth inhibition may be ideal to avoid potential adverse interactions.

[0134] The present maraging steel alloys may be used in a variety of applications, such as improved tooling for injection molding for forging, through-hardening gear materials, optimized-shape high-strength maraging steel components, and other such applications. Furthermore, the present disclosure may be applicable to a variety of maraging steel alloy systems, such as those specifically discussed herein and other systems.

[0135] Figure 14 illustrates an exemplary embodiment of the present application utilizing the functionalized maraging steel alloy 120 of the present disclosure in an aircraft 195. As shown in Figure 14, the maraging steel alloy 120 can be incorporated into the structure of the aircraft 195 to provide increased strength to such structure. The functionalized maraging steel alloy 120 is not limited to the locations shown in Figure 14 and can be incorporated into various components of the aircraft 195 to provide improved strength to such components.

[0136] The following examples are provided to illustrate one or more embodiments of the invention and should not be construed as limiting the invention.

[0137] Example FIGS. 10A-10C illustrate various microstructures, including an equiaxed microstructure, according to embodiments of the present disclosure. In particular, FIGS. 10A and 10B are images from a scanning electron microscope (SEM). FIG. 10A illustrates columnar grains 185 of a typical M300-type alloy fabricated by additive manufacturing. As shown in FIG. 10A, the resulting microstructure has a highly columnar grain structure. FIG. 10B illustrates an M350-type composition having 1 vol.% CeO as a grain growth inhibitor 110, which results in equiaxed grains 135 resulting in the equiaxed microstructure 185. FIG. 10C illustrates the characteristic scallop pattern 186 seen in both FIGS. 10A and 10B, indicating that the material was created by additive manufacturing. As seen in FIGS. 10A-10C, the additive manufacturing process produces a unique microstructure with a weld-like pattern.

[0138] Examples of possible grain growth inhibitors are shown in Figures 11A through 11F. In particular, Figures 11A through 11F are images from a scanning electron microscope (SEM). Figure 11A shows CL50, a compositional variant of M300. Figure 11B shows Fe377, a compositional variant of M350. Figure 11C shows Fe378, a compositional variant of M350 with small additions of Nb and B dissolved in the alloy. Figure 11D shows Fe377 with CeO2 as the grain growth inhibitor 110. Figure 11E shows Fe377 with TiN as the grain growth inhibitor 110. Figure 11F shows Fe377 with TiB2 as the grain growth inhibitor. As shown in Figures 11A through 11C, maraging steel alloys formed without grain growth inhibitors have highly columnar grain structures. In comparison, the functionalized maraging steel alloy 120, as shown, for example, in Figures 11D-11F, results in an equiaxed microstructure.

[0139] Heat treatment response data obtained for an exemplary functionalized maraging steel alloy 120 is shown in Figure 12. As shown in Figure 12, by incorporating the grain growth inhibitors 110, 115 of the present disclosure into the functionalized maraging steel alloy 120, for example, in additive manufacturing, high hardness values ​​can be achieved. The functionalized maraging steel alloy 120 of Figure 12 is a compositional variant of M350. The results shown in Figure 12 demonstrate that high hardness is possible in the functionalized maraging steel alloy 120 of the present disclosure produced using additive manufacturing.

[0140] In some embodiments, a 480°C heat treatment may be recommended, however, the functionalized maraging steel alloy 120 of the present disclosure may have peak strength when aged at a lower aging temperature, as shown in Figure 12. Aging may result in high strength in the functionalized maraging steel alloy 120.

[0141] In this detailed description, reference has been made to several embodiments and to the accompanying drawings which show, by way of example, certain exemplary embodiments of the invention. These embodiments have been described in sufficient detail to enable those skilled in the art to practice the invention, and it should be understood that modifications may be made by those skilled in the art to various embodiments of the present disclosure.

[0142] While the methods and steps described above show certain events occurring in a particular order, those skilled in the art will recognize that the order of certain steps can be changed and that such changes are consistent with certain variations of the present invention. Furthermore, certain steps may be performed simultaneously in parallel processes where possible, or may be performed sequentially.

[0143] The above-described embodiments, variations, and illustrations should provide an indication of the utility and versatility of the present invention. Other embodiments that do not necessarily provide all of the features and advantages described herein may be utilized without departing from the spirit and scope of the present invention. Such variations and modifications are deemed to be within the scope of the present invention as defined by the claims.

[0144] Having described exemplary embodiments at a high level, various configuration designs for performing various exemplary operations are provided below.

[0145] Furthermore, the present disclosure includes embodiments according to the following clauses:

[0146] Item A1. A maraging steel alloy comprising: a base maraging steel alloy; a grain growth inhibitor dispersed throughout the base maraging steel alloy; and optional strengthening elements, wherein the base maraging steel alloy is surface functionalized with the grain growth inhibitor.

[0147] Item A2. The maraging steel alloy of paragraph A1, wherein the base maraging steel alloy comprises aluminum, cobalt, molybdenum, nickel, titanium, or a combination thereof, and has a tensile strength greater than 1300 MPa.

[0148] Paragraph A3. A maraging steel alloy according to paragraphs A1 or A2, wherein the grain growth inhibitor comprises titanium, zirconium, boron, aluminum, tantalum, tungsten, carbon, niobium, cerium, or a combination thereof, as a pure metal, oxide, hydride, carbide, nitride, intermetallic compound, boride, or a combination thereof.

[0149] Paragraph A4. The maraging steel alloy of any one of paragraphs A1 through A3, wherein the strengthening element includes nickel, aluminum, cobalt, chromium, molybdenum, carbon, manganese, niobium, zirconium, titanium, or a combination thereof.

[0150] Item A5. The maraging steel alloy of any one of paragraphs A1 to A4, wherein the grain growth inhibitor comprises TiB2, CeO2, TiN, NbC, or a combination thereof.

[0151] Item A6. The maraging steel alloy according to any one of Items A1 to A5, wherein the grain growth inhibitor comprises about 0.01% to about 10% by volume of the maraging steel alloy.

[0152] Item A7. The maraging steel alloy of any one of items A1 through A6, comprising an equiaxed microstructure.

[0153] Paragraph A8. The maraging steel alloy of any one of paragraphs A1 to A7, wherein the equiaxed microstructure comprises a plurality of grains less than 1 mm in diameter.

[0154] Paragraph A9. The maraging steel alloy of any one of paragraphs A1 to A8, wherein the equiaxed microstructure comprises a uniform grain pattern along the x and y directions.

[0155] Paragraph A10. The maraging steel alloy of any one of paragraphs A1 to A9, wherein the equiaxed microstructure forms a scalloped pattern.

[0156] Item A11. An aircraft comprising the maraging steel alloy of any one of items A1 through A10.

[0157] Item B1. A method for producing a maraging steel, comprising the steps of mixing a base maraging steel alloy with a grain growth inhibitor to provide a maraging steel mixture, melting the maraging steel mixture, and solidifying the maraging steel mixture to form an equiaxed microstructure.

[0158] Item B2. The method for producing a maraging steel according to Item B1, wherein the step of solidifying the maraging steel mixture includes solidifying a first layer of the maraging steel mixture along a single axis and then solidifying an adjacent layer of the maraging steel mixture along the same axis.

[0159] Item B3. A method for producing a maraging steel according to items B1 or B2, wherein the base maraging steel alloy is present as a powder when mixed with the grain growth inhibitor.

[0160] Item B4. A method for producing a maraging steel according to any one of Items B1 to B3, wherein the base maraging steel alloy and the grain growth inhibitor have a lattice strain of less than 5%.

[0161] Item B5. A method for producing a maraging steel according to any one of Items B1 to B4, wherein the base maraging steel alloy and the grain growth inhibitor have an atomic density difference of less than 25%.

[0162] Item B6. A method for producing a maraging steel according to any one of items B1 to B5, wherein the base maraging steel alloy comprises aluminum, cobalt, molybdenum, nickel, titanium, or a combination thereof, and a tensile strength of greater than 1300 MPa.

[0163] Item B7. A method for producing a maraging steel according to any one of Items B1 to B6, wherein the grain growth inhibitor comprises titanium, zirconium, boron, aluminum, tantalum, tungsten, carbon, niobium, cerium, or a combination thereof, as a pure metal, oxide, hydride, carbide, nitride, intermetallic compound, boride, or a combination thereof.

[0164] Item B8. A method for producing a maraging steel according to any one of items B1 to B7, wherein the base maraging steel alloy further comprises strengthening elements including nickel, aluminum, cobalt, chromium, molybdenum, carbon, manganese, niobium, zirconium, titanium, or a combination thereof.

[0165] Item B9. A method for producing a maraging steel according to any one of items B1 to B8, wherein the grain growth inhibitor comprises TiB2, CeO2, TiN, NbC, or a combination thereof.

[0166] Item B10. A method for producing a maraging steel according to any one of Items B1 to B9, wherein the grain growth inhibitor comprises about 0.01% by volume to about 10% by volume of the maraging steel alloy.

[0167] The word "exemplary," as used herein, is intended to mean "serving as an example, instance, or illustration." Any approach described herein as "exemplary" is not necessarily preferred or advantageous over other approaches.

[0168] As used in this specification and the appended claims, the singular forms "a," "an," and "the" include plural references unless the context clearly indicates otherwise. For example, a reference to a "reinforcing member" includes a plurality of such reinforcing members unless the context clearly indicates otherwise.

[0169] As used in this specification and the appended claims, references to "on" include both embodiments in which one component is disposed directly on another component as well as embodiments in which one or more intervening layers or elements are disposed between the components. [Explanation of symbols]

[0170] 100 base maraging steel alloy 110 Crystal growth inhibitors 115 Crystal growth inhibitors 120 Functionalized maraging steel alloy 130 Reinforcement elements 132 Maraging Steel Mixture 135 Equiaxed particles Surface of 140 base maraging steel alloy 145 Melt 146 First Layer 147 Second Layer 150 solid materials 155 Crystal growth inhibitor layer 160 Contaminant surface layer 170 Melt 175 Dispersoid 180 Coating, melt 181 First crystal growth inhibitor layer 182 base maraging steel alloy layer 183 Second crystal growth inhibitor layer 185 Equiaxed microstructure 186 Scallop Pattern 195 Aircraft

Claims

1. a powdered base maraging steel alloy (100); a grain growth inhibitor (110, 115) dispersed in the base maraging steel alloy (100); It contains the base maraging steel alloy (100) is surface functionalized with the grain growth inhibitor (110, 115) dispersed along the surface of the base maraging steel alloy (100); the crystal growth inhibitor (110, 115) has an average maximum particle size of 1 nanometer or more and less than 100 nanometers; the grain growth inhibitor (110, 115) comprises titanium, zirconium, boron, aluminum, tantalum, tungsten, niobium, cerium, or a combination thereof, as a pure metal, an oxide, a hydride, a carbide, a nitride, an intermetallic compound, a boride, or a combination thereof; Maraging steel alloy (120).

2. 10. The maraging steel alloy (120) of claim 1, wherein the base maraging steel alloy (100) comprises aluminum, cobalt, molybdenum, nickel, titanium, or a combination thereof and has a tensile strength greater than 1300 MPa.

3. 3. The maraging steel alloy (120) of claim 1 or 2, further comprising a strengthening element (130), wherein the strengthening element (130) comprises nickel, aluminum, cobalt, chromium, molybdenum, carbon, manganese, niobium, zirconium, titanium, or a combination thereof, and wherein the base maraging steel alloy has a tensile strength greater than 1300 MPa.

4. 1. A method of manufacturing a maraging steel article, comprising: mixing a powdered base maraging steel alloy (100) with a grain growth inhibitor (110, 115) to provide a maraging steel mixture (132); Melting the maraging steel mixture (132); solidifying the maraging steel mixture (132) to form an equiaxed microstructure (185); Including, mixing a powdered base maraging steel alloy (100) with a grain growth inhibitor (110, 115) to provide a maraging steel mixture (132), the base maraging steel alloy (100) is surface functionalized with the grain growth inhibitor (110, 115) dispersed along the surface of the base maraging steel alloy (100); The crystal growth inhibitor (110, 115) has an average maximum particle size of 1 nanometer or more and less than 100 nanometers; A method for producing a maraging steel article.

5. 5. The method of manufacturing a maraging steel article of claim 4, wherein the base maraging steel alloy (100) and the grain growth inhibitors (110, 115) have a lattice strain of less than 5%.

6. 6. The method of manufacturing a maraging steel article according to claim 4 or 5, wherein the base maraging steel alloy (100) and the grain growth inhibitors (110, 115) have an atomic density difference of less than 25%.

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